Radio transmission systems, whether terrestrial radio relay systems or satellite transmission systems, make it possible to install new radio links, or to add to existing ones, very quickly. In such radio transmission systems, on one hand, the frequency spectrum available should be utilized as optimally as possible, and on the other hand, the radio transmission should be as resistant as possible to interfering influences (fading caused by weather, frequency-selective fading, interference by reflections, interference by neighboring radio cells, interference by other radio communication services, amplitude and phase distortions).
The demand for optimal frequency-spectrum utilization can be realized with point-to-multipoint radio transmission systems (radio relay, satellite radio). A point-to-multipoint radio relay system is explained in Mikrowellen-Magazin (Microwave Magazine), Vol. 10, No. 6, 1984, pp. 629, 630. According to that, utilization of the frequency band of the radio frequency channel available can be improved by usage of the necessary frequency band only as needed. The communication between the central station and the individual subscriber stations can be effected either through multiple access in frequency-division multiplexing (FDMA), in time-division multiplexing (TDMA), in code-division multiplexing(CDMA), in space-division multiplexing (SDMA), or in mixed forms of these access methods, it being possible to assign the frequency channels, the time slots, the spread codes and the spatial antenna lobes depending on the need of the subscribers.
In a point-to-multipoint radio relay system according to the German Patent Application No. 44 26 183, the transmission capacity is adapted flexibly to the need of the subscribers by adjusting the band width of the individual transmission channels to the data-transmission rate required in each case by the individual subscribers. Provision is also made for variable adjustment of the modulation type and the modulation depth (e.g. N-PSK, where N=2 . . . 16, or M-QAM, where M=4 . . . 256) to the individual transmission channels.
Channel coding represents a first aid for improving the resistance to interference. A further increase in interference resistance can be achieved by spectrum spreading and/or frequency hopping, as well as by orthogonal multi-carrier transmission according to a fixed, predefined scheme.
European Patent Application No. 0 719 062 describes a point-to-multipoint radio transmission system in which provision is made for a variable band-width allocation (band width on demand) between a central station and the individual subscribers. In this conventional transmission system, the signal transmission is based on a combination of code-division multiplexing (CDMA) and time-division multiplexing (TDMA). This European Patent Application discloses nothing about the operation of transmission and receiving devices of the subscribers and of the central station in response to a change in the band-width allocation.
Another conventional point-to-multipoint signal-transmission system is described in PCT International Patent Publication No. WO 95/34149, which includes no measures which permit a flexible adaption of transmission capacities to connected subscribers without interruption and interference.
At this point, the object of the present invention is to specify a point-to-multipoint radio transmission system of the type named at the outset, which both allows a flexible allocation and adaptation of transmission capacities to the connected subscriber stations, free of interruption and interference, on one hand, and, on the other hand, makes the radio transmission as resistant as possible to interfering influences (fading caused by weather, frequency-selective fading, interference by reflections, interference by neighboring radio cells, interference by other radio communication services, amplitude and phase distortions).